Sensors for radiation of different types are known for different applications, such as light sensors, x-ray sensors and others. Depending upon the wavelength and rate of change of radiation to be sensed, different types of sensors are appropriate. One particular field of light sensing is confocal microscopy.
The concept behind confocal microscopy as used in fluorescence microscopy is as follows. Fluorescent dye molecules can be attached to specific parts of the biological sample of interest. When excited by a suitable wavelength of light these markers fluoresce so that only those parts are seen in the microscope. The fluorescence is usually stimulated by a laser or lasers and is detected by a suitable sensor, usually a photomultiplier tube. The sample is scanned by the illumination/detector to build up an image of the fluorescence across the sample. It is necessary to place a very small aperture (pin hole) within the optical path to prevent light from unwanted regions of the sample from being detected. One arrangement of a typical confocal microscopy system is illustrated in FIG. 1.
The laser scanning microscopy device of FIG. 1 comprises a laser 10 arranged to illuminate a sample 12 via a dichroic mirror and two scanning mirrors 16, 18. The scanning mirrors cause the laser beam to scan the surface of the sample 12. Imaging optics 20 are arranged so that the scanning spot of light in the sample is imaged onto a pin hole 22. This arrangement is referred to as a confocal arrangement and ensures that only fluorescence from a particular spot being scanned reaches a detector 24. Any light from a depth within the sample that is not at the focus of the optics will not be focussed exactly on the pin hole (it will form a pattern larger than the pin hole) and so will not pass to the detector 24. The confocal arrangement thus allows different portions of a sample to be sampled over time (as the beam scans) and so this is a form of time resolved microscopy.
In the example of confocal microscopy, it is important that the light level can be sampled rapidly so that individual spots are sampled as the laser scans the surface of the sample. There are similar time constraints in other applications.
We have appreciated that a type of sensor known as a Silicon Drift Detector has certain advantages of speed in applications such as microscopy. The concept of the silicon drift detector was proposed in 1983 by E. Gatti, P. Rehak, “Semiconductor Drift Chamber—An Application of a Novel Charge Transport Scheme”, Nucl. Instr. and Meth. A 225, 1984, pp. 608-614. It consists of a volume of fully depleted high-resistivity silicon, in which an electric field with a strong component parallel to the surface drives electrons generated by the absorption of ionising radiation towards a small sized collecting anode. The electric field is generated by a number of increasingly reverse biased field strips (creating p-n diodes) covering one surface of the device. The concept is shown in FIG. 17.
Various refinements have been proposed including a concentric ring arrangement where the sense node is in the centre of the device. These devices find particular application in the field of x-ray spectroscopy. The structure is almost always based on refinements of the arrangement shown in FIG. 17 in which an appropriate electric field is applied via the diode contacts to sweep all the charge out of the device.
Variations on known Silicon Drift Detectors are known in various prior published documents.
U.S. Pat. No. 4,688,067 describes the operation of the silicon drift detector. This shows how biases applied to the p+ contacts can be used to sweep charge to a sense node.
WO 2006/012764 discloses a type of variable aperture sensor with variable size based on parallel electrodes arranged to vary an effective “slit” size.
U.S. Pat. No. 4,837,607 improves on the electrode arrangement.
EP 0383389 describes a modification to the SDD whereby the signal generated through the p+ contacts is used to time the arrival of the incident radiation and thus gain positional information.
U.S. Pat. No. 6,249,033 B1 describes a complication of the basic SDD to obtain positional information without the need for timing.
US 2004/0149919 A1 shows a modification to the SDD to improve uniformity of response etc.
U.S. Pat. No. 6,794,654 B1 describes a module of SDDs.
US 2005/0173733 A1 describes how to make contact to the sensing node in the middle of a concentric ring arrangement.
WO 2006/053938 A1 describes a very specific modification to the SDD in order to prevent surface leakage effects.